1. Field of the Invention
The present invention relates to enhancing color reproducibility in an imaging device. More particularly, the present invention relates to an enhancing visibility of an image output on a display unit of a mobile terminal when viewed outdoors.
2. Description of the Related Art
Recently, a variety of digital devices that meet the diverse needs of consumers have come into wide use. These digital devices can range from those having large-sized displays, such as computers and digital televisions (TVs), to those having small-sized displays, such as mobile phones, personal digital assistants (PDAs), and portable multimedia players (PMPs).
Unlike large-sized digital devices, small-sized digital devices, such as mobile phones, can be used both fixed settings, and in various locations with a range of ambient conditions, such as in dark rooms or in outdoor environments under direct sunlight. Therefore, there is along-felt need in the art to develop a method of optimally displaying an image by providing appropriate brightness and contrast of an image displayed under a given external luminance. For example, when an external luminance is high, the brightness and contrast of the displayed image must be increased.
However, there are difficulties in determining how to adjust the lightness and the contrast of a displayed image according to changes in external luminance. The same is true for adjustment of the chroma of the displayed image according to such changes in the brightness and the contrast. Accordingly, various technologies for solving the visibility deterioration problem in digital devices, including displays, according to external luminance information have been proposed without success.
Meanwhile, a user may change the lightness of a display device according to external illumination while watching images displayed by the display device. In this case, the visibility of images displayed by the display device may deteriorate because the change of the lightness of the display device does not reflect various image properties, e.g. lightness, luminance, contrast, chroma, etc., that affect the visibility of the display device. Also, when the lightness of the display device is changed, the power consumption of a display device is rarely taken into consideration. When the recent tendency for many image display devices to be compact-sized and portable is considered, it is highly important to develop image display devices that can ensure the visibility of images while consuming less power.
In many cases, mobile terminals are typically used outdoors. When an ambient light is greater than a brightness (i.e. luminance or lightness) of the display unit of a mobile terminal, there occurs a drawback that visibility deteriorates enough to make it difficult for a user to identify an output screen of the display unit. When the display unit is a transmissive liquid crystal display (LCD) using only a backlight source as a light source, the drawback of visibility deterioration worsens due to a limitation in the brightness of the backlight used as the light source.
Since the conventional technologies of enhancing the brightness of the backlight cause large power consumption, they are unsuitable for mobile terminals or the like, which have a limitation in supplying power and are used mainly in outdoors environments, because most mobile terminals or the like use batteries as power sources.
The Commission International De L'Eclairage (CIE) is an international organization concerned with solving the above-mentioned problem. The CIE has proposed CIE tristimulus values based on an RGB function which is a function of three primary colors of light.
The RGB function may form at least one negative value. Therefore, when an imaging device is realized by using the RGB function, there are problems in that an algorithm is complicated and errors increase due to the negative value. In contrast, according to the tristimulus values based on XYZ, since negative values are converted into positive values and are then used, the problem of the RGB function can be solved. However, in the case of an LCD monitor used for a portable imaging device and having deteriorated luminance and chromaticity, in comparison with the conventional CRT, there is a problem in that large noise and distortion are caused during the quantization and the digital post-processing.
Another problem with the previously proposed solutions is that when a conversion curve is used in the RGB space, a chromaticity value may be changed. The RGB space is a color space in which it is difficult for one color to be continuously maintained. When a change is made to the same value, or when enhancement is made to a specific brightness, a chromaticity value is changed, so that a desired color cannot be maintained, which is a problem.
In comparison with the RGB space, the YUV space is effective in separately applying the chromaticity value and brightness, but has a disadvantage in that the uniformity is reduced. That is, when brightness increases by one, there must be an increase by the same level over all the sections. However, in reality, there is not such an increase throughout the sections. A space for solving such a problem is referred to as a CIELAB space (CIELAB being a mathematical derivative of CIE XYZ (1931) that describes colors using three synthetic primaries: L* (which indicates Lightness), a* (which indicates red-greenness), and b* (which indicates yellow-blueness)). The CIELAB space is also called an “equivalent visual coordinate system,” and has a disadvantage in that the processing procedure is somewhat complex. The calculation procedure for processing the CIELAB is as follows.
Converting RGB Values into XYZ Tristimulus Values
First, after luminance and chromaticity values are measured with approximately 216 different color patches given to a display unit as an input, an estimation function is created, and XYZ values are estimated when certain RGB values are input.
Second, XYZ values are converted into Lab values by the following equation.L*=116×(Y/Yn)1/3−16a*=500×((X/Xn)1/3−(Y/Yn)1/3) XnYnZn:b*=200×((Y/Yn)1/3−(Z/Zn)1/3) white reference stimulus values
Dividing Lab into Lightness, Chroma, and Hue
      C    =                            a          2                +                  b          2                          H    =          arctan      ⁡              (                  b          a                )            
The Lab value is used as an “L” value, without change.
Changing “L” in order to enhance lightness and changing “C” in order to enhance chroma
However, the aforementioned methods requires a large amount of resources to be consumed when the RGB of an input image are converted into XYZ. Especially, in the case of a small-sized mobile display unit, since the same input does not result in the same luminance and chromaticity values, considerable measurement error is included, so that a problem is caused.